1. Field of the Invention
The present invention relates to ultrasonic transducers such as used for diagnostic imaging, non-destructive material testing and treatment of human organs and to methods for making such transducers.
2. Background of the Invention
Many types of transducers have been developed for a variety of imaging applications. Ultrasonic devices such as single element, annular arrays, one-dimensional arrays, 1.5 dimensional linear arrays and two-dimensional (2D) matrix arrays are examples of devices used as medical transducers. Recently, matrix shaped ultrasonic transducers designed for three-dimensional (3D) imaging capabilities have been introduced into the marketplace. Reference is made, for example, to U.S. Pat. No. 5,732,706 (White et al) which discloses a piezoelectric matrix array transducer connected into an integrated circuit. For a matrix transducer, the active surface is generally square shaped and the elements are arranged in a N by N matrix fashion wherein each transducer is individually addressed so any focal depth can be electronically controlled. As disclosed in U.S. Pat. No. 5,894,646 (Hanafy et al), typical manufacturing and interconnection methods for matrix transducers are based on the extension of existing manufacturing processes developed for one dimensional linear array transducers. However, these manufacturing methods have led to compromises in performance and to complexity in fabrication. Typically, a single flexible circuit or printed circuit board is used to connect the individual elements of a transducer array to a transducer cable. The use of this technique for a matrix array is not practical because the number of elements involved is significantly higher. For example, there may be 64 to 256 elements in a standard transducer, whereas a matrix transducer has up to 10,000 elements and potentially even more. Standard flexible circuits do not have the density required for this number of elements and thicker multilayer printed circuit boards, such as disclosed in the U.S. Pat. No. 5,855,049 (Corbett et al) degrade the performance of the transducer when placed between the piezoelectric material and the backing material.
According to the requirements of ultrasonic imaging, array transducers must exhibit acceptable acoustic performance to enable the system to provide high quality images. In general, matrix transducers must yield a 2D ultrasonic image quality approaching that obtained with linear array transducers, which means that individual transducer elements of the matrix must be designed to operate substantially identically to conventional transducers.
Ultrasonic transducers are designed to operate in a forward direction, meaning that, in medical applications, the ultrasonic transducer is pointed toward the organ to be imaged. As a consequence, such transducers are constructed to enhance sound propagation from the front face thereof and to minimize sound propagation from the back side thereof. The acoustic energy or reflections emanating from the rear face of the transducer is minimized by the use of a backing material.
Referring to FIG. 1, there is shown, in a cross-section, a typical prior art construction of a linear ultrasonic transducer. The transducer comprises the following components: a focusing lens 3, one or more impedance matching layers or members 2, a piezoelectric layer or member 1, an interconnect layer or member 5 and a sound absorbing backing layer or member 4. Typically, interconnect layer 5 is made of, or formed by, a flexible circuit or printed circuit board. A typical matrix array is shown in cross-section in FIG. 2. The matrix array is similar to the standard transducer except for the absence of a focussing lens and a much thicker interconnection layer 6, and includes an impedance matching layer 8, a further matching layer 9, a piezoelectric layer 7 and a sound absorbing backing layer 10. As illustrated in FIGS. 1 and 2, the interconnect layer 5 or 6 is usually placed at the interface between backing layer 4 or 10 and the piezoelectric material 1 or 7.
Numerous techniques of 2D connection have been developed during recent years, but none of these has provided a satisfactory solution to the problems sought to be solved by the present invention. These prior art attempts sometimes include the use of so-called “visible” multi-layer circuits which dramatically degrade the transducer response from reflections and sometimes use a connecting method so complicated that the resulting transducer is simply too expensive and unreliable to manufacture.
A further patent of interest here is U.S. Pat. No. 5,267,221 (Miller). This patent discloses an acoustic transducer assembly having a one or two dimensional array of transducer elements, an electrical circuit element such as a printed circuit board and a backing block for interconnecting transducer elements to corresponding contacts or traces of the board. Individual contacts for each transducer element are provided on the top and bottom surfaces of the backing block. The backing block comprises acoustic attenuating material having conductors extending therethrough which interconnect each transducer element to a corresponding circuit contact. The conductors are implemented using thin conductors, conducting fibers or foils, and multiple thin conductors or conducting fibers or foils may be used for each transducer element. This method however requires complicated tooling and methods to align the individual conductors. Furthermore the conductors are so thin as to make it difficult to achieve a reliable contact, and the conductors can collapse if excessive force is exerted on the backing.
There exists a need for a new way of interconnecting individual matrix transducer elements so as to provide a high density of interconnects without compromising the acoustic performance of the transducers.